legal key date (ie, keyage children, between eight and nine years in third grade) exhibited a
linear physical fitness development in the ninth year of life. In contrast, children who were
enrolled with a delay (ie, older-than-keyage children [OTK], between nine and ten years in
third grade) exhibited a lower physical fitness compared to what would be expected for their
age. In these studies, cross-sectional age differences within third grade and timing of school …